Quantifying velocity and turbulence structure in depositing sustained turbidity currents across breaks in slope   总被引：2，自引：0，他引：2  

THOMAS E. GRAY  JAN ALEXANDER  MIKE R. LEEDER 《Sedimentology》2005,52(3):467-488

Two-dimensional experiments investigating sediment transport and turbulence structure in sustained turbidity currents that cross breaks in slope are presented as analogue illustrations for natural flows. The results suggest that in natural flows, turbulence generation at slope breaks may account for increased sand transport into basins and that the formation of a hydraulic jump may not be necessary to explain features such as the occurrence of submarine plunge pools and the deposition of coarser-grained beds in the bottomsets of Gilbert-type fan deltas. Experimental flows were generated on 0°, 3°, 6° and 9° slopes of equal length which terminated abruptly on a horizontal bed. Two-component velocities were measured on the slope, at the slope break and downstream of the slope break. Flows were depositional and non-uniform, visibly slowing and thickening with distance downstream. One-dimensional continuous wavelet transforms of velocity time series were used to produce time-period variance maps. Peaks in variance were tested against a background red-noise spectrum at the 95% level; a significant period banding occurs in the cross-wavelet transform at the slope break, attributed to increased formation of coherent flow structures (Kelvin–Helmholtz billows). Variance becomes distributed at progressively longer periods and the shape of the bed-normal-velocity spectral energy distribution changes with distance downstream. This is attributed to a shift towards larger turbulent structures caused by wake stretching. Mean velocity, Reynolds shear stress and turbulent kinetic energy profiles illustrate the mean distribution of turbulence through the currents. A turbulent kinetic energy transfer balance shows that flow non-uniformity arises through the transfer of mean streamwise slowing to mean bed-normal motion through the action of Reynolds normal stresses. Net turbulence production through the action of normal stresses is achieved on steeper slopes as turbulence dissipation due to mean bed-normal motion is limited. At the slope break, an imbalance between the production and dissipation of turbulence occurs because of the contrasting nature of the wall and free-shear boundaries at the bottom and top of the flows, respectively. A rapid reduction in mean streamwise velocity predominately affects the base of the flows and steeper proximal slope flows have to slow more at the break in slope. The increased turbulent kinetic energy, limited bed-normal motion and strong mixing imposed by steep proximal slopes means rapid slowing enhances turbulence production at the break in slope by focusing energy into coherent flow structures at a characteristic period. Thus, mean streamwise slowing is transferred into turbulence production at the slope break that causes increased transport of sediment and a decrease in deposit mass downstream of the slope break. The internal effects of flow non-uniformity therefore can be separated from the external influence of the slope break.  相似文献   

Giant flute-like scour and other erosional features formed by the 1929 Grand Banks turbidity current   总被引：1，自引：0，他引：1  

ALEXANDER N. SHOR  DAVID J. W. PIPER  JOHN E. HUGHES CLARKE  LARRY A. MAYER 《Sedimentology》1990,37(4):631-645

Erosional features on the floor of Eastern Valley of the Laurentian Fan, in 2800 m water depth, have been mapped with SeaMARC I side-scan sonar images and Seabeam multi-beam echo-soundings, and were directly observed during a dive with the deep submersible Alvin. The most spectacular feature is a 100-m-deep flute-shaped scour, more than 1 km long. The surrounding valley is floored by an unconsolidated coarse conglomerate, which was moulded into transverse bedforms by the turbidity current that was triggered by the 1929 Grand Banks earthquake. Direct observations and seismic-reflection profiles show that the flute-shaped scour cuts through this conglomerate and into Plio-Pleistocene valley-floor sediments, thereby exposing a section through the 1929 deposit. Application of the Allen defect theory suggests that the flute is unusually deep because general channel-floor erosion was inhibited by the conglomerate veneer. Valley-floor channels typically 1 km wide and 10m deep contain series of closed depressions that occasionally deepen to 30 m. These are also interpreted as erosional scours, analogous to pools cut on the beds of bedrock rivers. The large flute was probably formed by detached flow enlarging an initial scour depression. Such scours probably play an important role in channel-floor erosion, increasing the volume of sediment transported by large turbidity currents.  相似文献   

Petrology and Geochemistry of Basalts from the Red Sea Axial Rift at 18? North     

EISSEN  JEAN-PHILIPPE; JUTEAU  THIERRY; JORON  JEAN-LOUIS; DUPRE  BERNARD; HUMLER  ERIC; AL'MUKHAMEDOV  ALEXANDER 《Journal of Petrology》1989,30(4):791-839

Detailed studies by submersible were carried out in the axialzone of the Red Sea Rift near 18?N during the Soviet Red Seaexpedition of the Oceanological Institute of the Academy ofSciences (December 1979–March 1980). The initial bathymetric,magnetic and seismic surveys established the general organizationof the symmetric tectonic steps (1–3) descending towardsthe axial rift. The 4–5 km wide inner floor of the riftwas explored during 21 dives. It is occupied by 100–300m high, young pillowed volcanoes, isolated or grouped to formelongated hills, frequently cut by open fissures except in thezone of most recent extrusion. The 42 samples collected are typical plagioclase ? olivine ?clinopyroxene ? spinel, more or less porphyritic mid-ocean ridgebasalts whose compositions were mainly controlled by polybaricfractionation of plagioclase, olivine and minor clinopyroxene.They have been separated into porphyritic and sub-aphyric groupsusing modes and mineralogical criteria. Mineral-liquid equilibria,crystal zonation, and modal proportions indicate some magmamixing but probably only of closely related magma batches withineach described group, as can occur inside a single magma chamber.Crystal accumulation is believed to have played a significantrole in only a few porphyritic samples. Three sub-groups (from less to more evolved; (a) FeO^*/MgO<1?22;(b) 1?16<FeO^*/MgO < 1?48; and (c)FeO^*/MgO>1?49) weredistinguished on the basis of glass and whole-rock major elementchemistry. Glass compositions follow the multisaturated cotectic-likecurve for MORB-type basalts and show a general evolution verycomparable to what is observed on the Mid-Atlantic Ridge near36?N, but arc less diverse than in the FAMOUS area. ⁸⁷Sr/⁸⁶Sr,¹⁴³Nd/¹⁴⁴Nd, and ²⁰⁶Pb/²⁰⁴Pb data for 4 samples show strongsimilarities to those from the Mid-Atlantic and East PacificRidges, and indicate no continental contamination despite thefact that they have been produced during recent continentalbreak-up and ocean opening. ²⁰⁶Pb/²⁰⁴Pb values, Th/Ta vs. Th/Tbcorrelations, and rare earth element patterns allow recognitionof three different groups of samples, indicating that the RedSea Rift near 18?N is fed by a heterogeneous mantle source.The chondrite-normalized LREE.  相似文献   

The origin and tectonic significance of asymmetrical meander-belts     

MIKE R. LEEDER  JAN ALEXANDER 《Sedimentology》1987,34(2):217-226

In the Hebgen lake area of SW Montana, the Madison and South Fork Rivers occupy the northeast sides of their respective meander-belts. The form of the abandoned meander-loops within these asymmetric meander-belts indicates that they were produced by gradual northeast migration of the active channels. This progressive migration can be related to tectonic tilting produced by regional extensional faulting at a high angle to the original channel-belt axis which ran down a gently sloping outwash cone. Asymmetric channel sandstone bodies in the rock record can be recognized by the preferred dip of successive lateral accretion units in meander loops. Together with soft sediment deformation, facies and thickness variations they may indicate tectonic deformation of the sedimentary surface.  相似文献   

Deformation and melting of steel projectiles in hypervelocity cratering experiments     

T. KENKMANN  G. TRULLENQUE  A. DEUTSCH  L. HECHT  M. EBERT  T. SALGE  F. SCHÄFER  K. THOMA 《Meteoritics & planetary science》2013,48(1):150-164

Abstract– We carried out hypervelocity cratering experiments with steel projectiles and sandstone targets to investigate the structural and mineralogical changes that occur upon impact in the projectile and target. The masses of coherent projectile relics that were recovered in different experiments ranged between 58% and 92% of their initial projectile masses. A significant trend between impact energy, the presence of water in the target, and the mass of projectile relics could not be found. However, projectile fragmentation seems to be enhanced if the target contains substantial amounts of water. Two experiments that were performed with 1 cm sized steel projectiles impacting at 3400 and 5300 m s^?1 vertically onto dry Seeberger sandstone were investigated in detail. The recovered projectiles are intensely plastically deformed. Deformation mechanisms include dislocation glide and dislocation creep. The latter led to the formation of subgrains and micrometer‐sized dynamically recrystallized grains. In case of the 5300 m s^?1 impact experiment, this deformation is followed by grain annealing. In addition, brittle fracturing and friction‐controlled melting at the surface along with melting and boiling of iron and silica were observed in both experiments. We estimated that heating and melting of the projectile impacting at 5300 m s^?1 consumed 4.4% of the total impact energy and was converted into thermal energy and heat of fusion. Beside the formation of centimeter‐sized projectile relics, projectile matter is distributed in the ejecta as spherules, unmelted fragments, and intermingled iron‐silica aggregates.  相似文献   

Crater morphology in sandstone targets: The MEMIN impact parameter study     

Anja DUFRESNE  Michael H. POELCHAU  Thomas KENKMANN  Alex DEUTSCH  Tobias HOERTH  Frank SCHÄFER  Klaus THOMA 《Meteoritics & planetary science》2013,48(1):50-70

Abstract– Hypervelocity (2.5–7.8 km s^?1) impact experiments into sandstone were carried out to investigate the influence of projectile velocity and mass, target pore space saturation, target‐projectile density contrast, and target layer orientation on crater size and shape. Crater size increases with increasing projectile velocity and mass as well as with increasing target pore space saturation. Craters in water‐saturated porous targets are generally shallower and larger in volume and in diameter than craters from equivalent impacts into dry porous sandstone. Morphometric analyses of the resultant craters, 5–40 cm in diameter, reveal features that are characteristic of all of our experimental craters regardless of impact conditions (I) a large central depression within a fragile, light‐colored central part, and (II) an outer spallation zone with areas of incipient spallation. Two different mechanical processes, grain fragmentation and intergranular tensile fracturing, are recorded within these crater morphologies. Zone (I) approximates the shape of the transient crater formed by material compression, displacement, comminution, and excavation flow, whereas (II) is the result of intergranular tensile fracturing and spallation. The transient crater dimensions are reconstructed by fitting quadric parabolas to crater profiles from digital elevation models. The dimensions of this transient and of the final crater show the same trends: both increase in volume with increasing impact energy, and with increasing water saturation of the target pore space. The relative size of the transient crater (in percent of the final crater volume) decreases with increasing projectile mass and velocity, signifying a greater contribution of spallation on the final crater size when projectile mass and velocity are increased.  相似文献   

Geochemistry and neodymium-strontium isotope signature of tektite-like objects from Siberia (urengoites,South-Ural glass)     

ALEXANDER DEUTSCH  MARKUS OSTERMANN  VICTOR L. MASAITIS 《Meteoritics & planetary science》1997,32(5):679-686

Abstract— We report Sr-Nd isotope parameters, rare earth element (REE), and major element data for isolated findings of tektite-like objects from western Siberia (urengoites, South-Ural glass), as well as for two indochinites. The latter were recovered in Vietnam and their overall geochemical characteristics equal those of other tektites from the indochinite subgroup of the Australasian strewn field. The three urengoites (～24 Ma) are extremely silica-rich (89 to 96 wt% SiC₂), and their REE abundances vary between 45 and 76 ppm. With La_N/Yb_N ranging from 7.6 to 10.4 and Eu_N/EU* between 0.69 and 0.75, their REE distribution patterns match that of average upper crust. The urengoites have present-day ?_Sr of +155 to +174 and ?_Nd ranging from ?18 to ?23. Their model ages in million years are: T^SruR = 1200 up to 4060 and T^NdcHUR = 1570 up to 2070. Data points for the urengoites plot colinearly in the Rb-Sr evolution diagram. The age corresponding to the slope is 183 ± 30 Ma (2s?), which is indistinguishable from the intercept age of 211 Ma in the T^Sr_UR vs. l/f_Rb diagram. Rubidium-strontium and Sm-Nd systematics of the urengoites indicate a heterogeneous precursor material, derived from Paleoproterozoic continental crust, which underwent Rb/Sr fractionation and partial Sr isotope homogenization in Jurassic times. Any relation between the urengoites and the Haughton impact crater, having within 2s? errors an identical age, can be excluded on the basis of isotope relationships and geochemical data. The only known South-Ural glass (～6.2 Ma) is characterized by intermediate SiO₂ (65 wt%), high Al₂O₃ (14 wt%) and CaO (12 wt%), and low FeO_TOT (0.4 wt%) contents. This unique tektite-like object contains 110 ppm REE displaying a steeply negative C1 normalized distribution with La_N/Yb_N of 17, and Eu_N/Eu¹ of 0.71. The Rb abundance (10 ppm) and Rb/Sr ratio are low, and combined with a “crustal” ⁸⁷Sr/⁸⁶Sr ratio of 0.722, yielding an unrealistic T^SruR age of 2.5 Ga. The Rb-Sr systematics imply a rather recent parent/daughter element decoupling. The T^NdCHUR age of the South-Ural glass is ～1690 Ma. Geochemical data suggest that urengoites and the South-Ural glass belong to two discrete groups of tektites, whose source craters remain to be discovered.  相似文献   

Experimental approach to generate shock veins in single crystal olivine by shear melting     

Falko Langenhorst  Jean‐Paul POIRIER  Alexander DEUTSCH  Ulrich HORNEMANN 《Meteoritics & planetary science》2002,37(11):1541-1553

Abstract— A shock experiment has been devised to produce large shear in a single crystal sample of olivine. The recovered sample exhibits macroscopic shear faults resembling shock veins in ordinary chondrites. Examination with transmission electron microscopy reveals a high density of dislocations in the bulk olivine. The shear faults appear as thin veins containing small grains of olivine and pockets of glass. The microstructure and composition of the material in the veins point to fractional crystallization of a melt. An order of magnitude calculation is consistent with the idea that the veins were produced by shear melting. These results support the view that shock veins in meteorites are the result of shear heating rather than of pressure heterogeneities.  相似文献   

Seasonal and spatial variability in soil CO₂ efflux rates for a central Siberian Pinus sylvestris forest     

OLGA SHIBISTOVA  JON LLOYD  SVETLANA EVGRAFOVA  NADJA SAVUSHKINA  GALINA ZRAZHEVSKAYA  ALMUT ARNETH  ALEXANDER KNOHL  OLAF KOLLE  E.-DETLEF SCHULZE 《Tellus. Series B, Chemical and physical meteorology》2002,54(5):552-567

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Sodium-, chlorine-rich mesostases in Chainpur (LL3) and Parnallee (LL3) chondrules     

J. C. BRIDGES  C. M. O'D. ALEXANDER  R. HUTCHISON  I. A. FRANCHI  C. T. PILLINGER 《Meteoritics & planetary science》1997,32(4):555-565

Abstract— Forty-six chondrules from Chainpur (LL3.4) and 39 chondrules and clasts from Parnallee (LL3.6) have been sectioned and searched for Na-, Cl-rich phases by electron probe microanalysis (EPMA). Oxygen isotopic compositions, I-Xe ages and ion probe data were also obtained on some of these chondrules. Sodium-, Cl-rich glass and microcrystalline sodalite (Na₄Al₃Si₃O₁₂Cl), nepheline (NaAlSiO₄), scapolite (Na₄Al₃Si₉O₂₄Cl) have been identified in 7% of the Chainpur and 8% of the Parnallee samples. These phases are present in chondrule mesostases or, in one case, the plagioclase of a barred-olivine chondrule. None of the chondrules contain >5 vol% Na-, Cl-rich phases. In the Chainpur chondrules, they originated through partial devitrification of silica-undersaturated, rare-earth-element-(REE), Na- and Cl-rich mesostases. Two processes have been identified that led to the formation of these mesostases. In two of the chondrules, which consist mainly of low-Ca pyroxene, the extended, metastable crystallization of low-Ca pyroxene created silica-undersaturated, REE-rich residua. Barium- and Cl-enrichments in nepheline and scapolite of one chondrule suggest that there was also an influx of alkalis and Cl during crystallization of the low-Ca pyroxene. Similarly, another one of the Chainpur chondrules, mainly composed of olivine phenocrysts, is markedly enriched in Cl (10 × OC). As there is no evidence of corrosive metasomatism in any of the chondrules, Cl- (and alkali) enrichment is believed to have occurred when they were still partially molten. The chondrules were derived from normal O-isotopic reservoirs, so the postulated influx of Ba, Na and Cl did not occur on an exotic parent body. Trace amounts of nepheline and sodalite, present in two Parnallee chondrules, crystallized from small Na-, Cl-, REE-rich residua following extended crystallization of anorthite. An I-Xe age of 5.0 Ma post-Bjurböle obtained on one of these Parnallee chondrules dates the crystallization of feldspathoid and, thus, formation of the chondrule.  相似文献